The present invention pertains to the technical field of inverter driven rotating electrical machines and more particularly, relates to a method of testing partial discharge in a rotating electrical machine driven by an inverter.
In recent years, variable speed running of a rotating electrical machine using an inverter has been put into practice extensively from the standpoint of economization of energy. But reports have been made on various problems raised in connection with insulation of the rotating electrical machine driven by the inverter (see the technical report of the Institute of Electrical Engineers of Japan, pp. 12 to 20, Vol. 739 (non-patent literature 1), for instance). The report is made on, for example, the fact that when an inverter surge voltage generated by ON/OFF of switching elements inside the inverter propagates through a cable to reach the end terminal of the rotating electrical machine, a mismatching of surge impedance between the cable and the rotating electrical machine becomes responsible for causing the inverter surge voltage to jump up to twice its original level at the rotating electrical machine end terminal. Another report has also been made on the fact that when a steep inverter surge voltage migrates into the rotating electrical machine, a high voltage is generated in the outlet coil of rotating electrical machine and/or across its internal winding turn. For these reasons, it is necessary that in the inverter driven rotating electrical machine, the machine should be so designed in insulation as to be durable against these inverter surge voltages and besides a product of rotating electrical machine should be given an inspection as to whether it has predetermined inverter surge insulation proof stress.
As one of methods for inspecting the insulation of inverter driven rotating electrical machine, an insulation inspection method using an impulse voltage has been carried out (see JP-A-2010-8199 and, IEC 60034-18-41 and IEC 60034-18-42 (non-patent literatures 2 and 3)). Particularly, in the non-patent literatures 2 and 3, an impulse voltage is applied to a rotating electrical machine to generate a voltage in the outlet coil of the rotating electrical machine or across its internal winding turn for the purpose of measuring a partial discharge. The literatures describe that this process uses an impulse voltage having the same voltage rise-up time as that of the inverter voltage applied to the rotating electrical machine upon start of inverter drive of the rotating electrical machine.